Black Beauties- Super Black Butterfly Scales

1
Black Beauties- Super Black Butterfly Scales

• Alison Sutton Fernandes
• 0225014

2
Why Butterflies?

• Butterflies have irridescent colours formed by
  photonic crystals.
• But what about the intense black areas on the
  wings?
• Wing scales with very low reflectance (gt0.5)
• Possibilities of emulating them with other
  materials.

http//www.thaishop4you.com/buttrfly_big_view/bf16
3.htm
3
Surface Reflections

• Any interface that involves a change in
  refractive index gives rise to surface
  reflections. Surfaces like black cardboard and
  paint, even though they appear black still
  reflect about 4.
• To a simple approximation, these surface
  reflections are governed by Fresnel equations.
  For air (ni) and chittin (nt)
• R ((nt-ni)/(ntni))2 ((1-1.56)/(11.56))2
• 4.8
• In butterfly scales, you get values as low as
  0.4.

4
The Role of the Butterfly Wing Scale

• The material the butterfly wing is made from,
  chitin, is effectively transparent. Yet when it
  adopts certain structures it can cause
  interference and diffraction of light rays to
  produce a range of colours.
• In the case of black scales the main role of the
  upper part of the wing scale appears to be to
  collimate the light- to transmit it to an
  absorbent membrane beneath, and minimise surface
  reflections. It is this part of the Scale I hoped
  to investigate.
• Begun investigations with 17 samples and a range
  of methods to see what different solutions there
  were and which were most effective.

5
High Resolution Optical Microscope
6
Typical Scale Structure

• The arrangement of scales on the wing resembles
  that of shingles on a roof. In most species two
  distinct layers are present- ground and cover
  scales.
• Typical scale dimensions are of the order 75micm
  by 200 micm. (scales come off as a fine dust).
  Underside tends to be plain and featureless,
  while interior and external visible top surface
  exhibit interesting microstructure.

7
Honeycomb Structure
8
(No Transcript)
9
Cross Ribs
10
Parides Hecuba

• Two butterflies of the Parides family (Hecuba and
  Rotuse) instead of honeycomb structure had
  microribbing extending across between the ridges,
  effectively blocking the inner layers below.
• Resulted in some of the lowest reflectances
  recorded.

11
Fractured Scales
12
Other Methods

• SEM
• Upper limit to resolution
• Difficulty seeing inner structure
• Hard to establish exact size of features
• Alternatives
• Embedded in Resin
• TEM

13
Cary SE Spectrophotometer

• Measures the reflectance of a sample over a range
  of wavelengths using an integrating sphere.
• Zero calibrated using a light trap extremely
  absorbing.
• Samples must be of sufficient size (limited to 5
  species).
• Beam must be carefully positioned.
• Scales easily lost.

14
(No Transcript)
15
(No Transcript)
16
Parides sesostris
17
Microspectrophotometer

• Spectral information from single scales.
• Problems
• Drifting dark current
• Limited integration time
• Very small area
• Surrounding reflections and extraneous light
• Lambertian assumption
• Equipment failure

18
Microspectrophotometer

• Attempted to Average Pixel Intensities

19
Conclusion

• Scales with the honeycomb structure were on
  average significantly less reflective than those
  with crossribbing.
• Suggests honeycomb more effective in minimising
  surface reflections and collimating light.
• The microribbing appeared even more effective.
• All scales exhibited extremely low reflectances

20
Why Colour and Black?

• Camouflage.
• Sex Attractant.
• An absorber, attenuator, or deflector for
  ultrasonics to defeat echolocations by bats.
• Signalling
• Identification- seen from a large distance,
  distinguishable from background.
• Eyespots- scare away predators.
• Effective use of light. When ample light is
  available to species, pigments are generally
  found. When light becomes scarce, more structural
  colour used (light is not lost and absorbed, but
  a lot reflected back).

21
Thermoregulation

• Butterflies bask to gain sufficient body
  temperatures for flight activity. (Berwaerts,
  2001)
• Butterflies with fully spread wings did warm more
  efficiently. (Heinrich, 1986)
• Descaled wings reached lower temperatures.
  (Berwaerts, 2001)
• Butterflies can develop different scales colours
  depending on the season they are born in.
• Behavioural factors, such as wing orientation
  seem more important. (Polycyn, 1986)
• The changes in reflectance are not great.
• Reflective in the infra-red region
• In some cases difficult to tell if behaviour
  adapts to wing colour or wing colour adapts to
  behaviour.

22
Other Research

• Moth eyes (Hutley et al)
• Minimise Surface Reflections
• Triangle like projections on surface
• Gradually decreasing diffractive index
• A similar type of structure is used to absorb
  sound wave in recording rooms without creating
  interference through reflections.
• Thin films also attempt this method, by layering
  films of slightly decreased refractive index to
  lower surface reflections.

23
Application

• Structures could be scaled for specific
  applications. You would create selective surfaces
  (since reflection in infra-red region is v.
  high).
• Basic computer modelling has already confirmed a
  peak below 1 for a simple honeycomb structure.
• Important to use nature as inspiration, not as
  blueprints.
• Needs of an individual organism likely to be very
  different form our own.

24
References

• Berwaerts, K., Van Dyck, H. Matthysen, E.,
  (2001), Effect of manipulated wing
  characteristics and basking posture on thermal
  properties of the butterfly Pararge aegeria,
  Journal of Zoology, 255(2), pp. 261-267
• Ghiradella, H., (1994), Structure of Butterfly
  Scales- Patterning in an Insect Cuticle,
  Microscopy Research and Technique, Apr 1 1994, 27
  (5), pp. 429-438
• Heinrich, B., (1986), Comparitive
  thermoregulation of four montane butterflies of
  different mass, Physiological Zoology, 59(6), pp.
  616-626.
• Lawrence, C. Large, M. C. J., (), Optical
  Biomimetics, ,
• Lewis, H. L., (1973), Butterflies of the World,
  Harrap, London
• Leo, B., (1999), Mysteries of a Butterfly Wing,
  Microscope, 47 (2), pp. 79-92.
• Polycyn, D. Chappell, M. A., (1986), Analysis
  of Heat Transfer in Vanessa Butterflies Effects
  of Wing Position and Orientation to Wind and
  Light, Physiological Zoology, 59(6), pp. 706-716

25
Acknowledgments

• OFTC Dr. Maryanne Large, Dr. Leon Poladian,
  Shelly Wickham
• Applied Physics Professor David McKenzie, Dr.
  Stephen Bosi
• EMU Tony Romeo, Dr. Ian Kaplin, Anne
  Simpson-Gomes
• Tamar Ziv, James Griffin